High electron mobility epitaxial substrate

ABSTRACT

A compound semiconductor epitaxial substrate for use in a strain channel high electron mobility field effect transistor, comprising an InGaAs layer as a strain channel layer  6  and AlGaAs layers containing n-type impurities as back side and front side electron supplying layers  3  and  9 , wherein an emission peak wavelength from the strain channel layer  6  at 77 K is set to 1030 nm or more by optimizing the In composition and the thickness of the strain channel layer  6.

TECHNICAL FIELD

The present invention relates to a compound semiconductor epitaxialsubstrate used for a pseudomorphic high electron mobility transistorcomprising a III-V compound semiconductor and to a method formanufacturing the same.

BACKGROUND ART

A high electron mobility field effect transistor (referred to as HEMThereinafter) has been used as an important component of a high-frequencycommunication instrument. A big feature of the HEMT is to have aselectively doped hetero structure comprising an electron supplyinglayer (a doped layer) for supplying electrons and a channel layerthrough which electrons run, these layers being made of differentmaterials. In this hetero structure, electrons supplied from n-typeimpurities within the electron supplying layer are pooled in a potentialwell formed at a channel side of a heterojunction interface due to adifference of electron affinity between materials constituting thehetero junction, and then a two-dimensional electron gas is formed.Since n-type impurities supplying electrons are present in the electronsupplying layer as described above and the electrons supplied from thelayer run through a high purity channel so that the ionized impuritiesand the electrons are spatially separated from each other, thetwo-dimensional electron gas within the channel is hardly scattered bythe ionized impurities, and consequently a high electron mobility isrealized.

Although the HEMT has usually been fabricated by using an epitaxialsubstrate in which thin film crystal layers respectively havingpredetermined electronic characteristics are laminated and grown on aGaAs single crystal substrate so as to have a predetermined structure,it has been required to precisely control the thin film crystal layerforming the HEMT structure on the order of monoatomic layer level forthe purpose of imparting a high electron mobility to the channel.Therefore, a molecular beam epitaxy (referred to as a MBE method,hereinafter) or a metalorganic chemical vapor deposition (referred to asa MOCVD method, hereinafter) has conventionally been used as a methodfor manufacturing an epitaxial substrate having a HEMT structure.

The MOCVD method, especially among other methods as described above forgrowing epitaxial substrates, uses an organometallic compound or ahydride of atomic species constituting an epitaxial layer as a sourcematerial and then pyrolyzes the source material on the substrate to growa crystal thereof, so that this method is applicable to a wide range ofsubstances and is not only suitable for precisely controlling thecrystal composition and thickness thereof but also capable of processinga large amount of substrates with favorable controllability, andconsequently this method has recently been used widely and commercially.

Although materials such as GaAs and AlGaAs have widely been used asIII-V compound semiconductors for these epitaxial substrates since thesematerials with any compositions can match the lattice constants thereofwith each other and allow for producing various hetero junctions whilekeeping good crystallinity thereof, it is necessary to further improvethe electron mobility in the channel layer in order to enhance theperformance of the HEMTs. Therefore, InGaAs has recently been used as amaterial for the channel layer instead of using GaAs, because InGaAs hasextremely excellent properties as the III-V compound semiconductor usedfor the hetero junction, that is, InGaAs is not only excellent in itselectron transporting characteristic but also capable of significantlychanging its energy gap in accordance with the In composition andfurther capable of effectively confining the two-dimensional electrons.In addition, AlGaAs or GaAs is known as a material to be combined withInGaAs.

However, the InGaAs cannot be lattice-matched with GaAs, so that it hasconventionally been impossible to obtain an epitaxial substrate havingsubstantial physical properties by using a InGaAs layer. However, it hassubsequently been found that a reliable hetero junction can be formedwithout unfavorably inducing a decrease in crystallinity such asproducing a dislocation even when a material with lattice misfit is usedprovided that the misfit is within a limit of elastic deformation, sothat there has practically been used an epitaxial substrate descriedabove.

An epitaxial growth substrate having a structure in which the abovedescribed InGaAs layer is used as a channel layer part of theconventional HEMT through which two-dimensional electrons flow has beenutilized for fabricating an electronic device which has a highermobility and is excellent in a noise characteristic compared with theconventional device. The HEMT, in which InGaAs layer is used for thechannel layer through which two-dimensional electrons flow, is referredto as a pseudomorphic high electron mobility transistor (hereinafter,referred to as a PHEMT).

A limit value of a thickness of the strain crystal layer in the abovedescribed lattice misfit material are given as a function of the straincrystal layer composition, and as for an InGaAs layer with respect to aGaAs layer for example, a Matthews' theoretical equation disclosed in J.Crystal Growth, 27 (1974) p. 118 and in J. Crystal Growth, 32 (1974) p.265, is known, and this theoretical equation has been found to beexperimentally correct as a whole.

JP-A-6-21106 discloses a technique for improving an electron mobility,in which an In composition of an InGaAs strain layer used as a channellayer of a PHEMT structure and a thickness of the InGaAs layer areoptimized by a certain relational expression, provided that a limitvalue of a thickness of the InGaAs layer given by the theoreticalequation is assumed to be an upper limit of the thickness range.

Since it is effective to additionally reduce the scattering oftwo-dimensional electrons caused by ionized impurities in order toimprove the mobility, a spacer layer which has the same material andcomposition as an electron supplying layer and to which any impuritiesare not added may be inserted between the electron supplying layer andthe channel layer. For example, Japanese Patent No. 2708863 discloses astructure for improving a two-dimensional electron gas concentration andan electron mobility, in which a spacer layer consisting of an AlGaAslayer and a GaAs layer is inserted between an InGaAs strain layer usedas a channel layer and an n-AlGaAs electron supplying layer of a pHEMTstructure and the growth condition is optimized.

When an InGaAs strain layer is used as a channel layer of the PHEMTstructure through which electrons run, it is possible to improve anelectron mobility at room temperature (300 K) compared with an epitaxialsubstrate of the HEMT structure in which a GaAs layer is used as achannel layer. However, the mobility at room temperature (300 K) as hasbeen reported before is 8000 cm²/V·s at the maximum, and thus it hasbeen difficult to achieve an electron mobility exceeding the abovedescribed value even in the case of a PHEMT structure epitaxialsubstrate in which an InGaAs strain layer is used as a channel layer.

If the structure disclosed in Japanese Patent No. 2708863 is adopted inorder to increase the electron mobility in the pHEMT structure epitaxialsubstrate, the electron mobility is improved with increases in athickness of the spacer layer, however, a concentration of thetwo-dimensional electron gas formed in the channel layer decreasesbecause a distance between the electron supplying layer and the channellayer becomes larger, and thus leads to an undesirable outcome.

In order to improve the electron mobility and the two-dimensionalelectron gas concentration in the channel layer simultaneously, it iseffective to increase an In composition of the channel layer and toincrease the layer thickness. This is because the increase in the Incomposition of the channel layer leads to a decrease in an effectivemass of electrons which travel through the channel layer for improvingthe electron mobility, and further to make a difference of conductionband energy between the electron supplying layer and the channel layerlarger, and consequently the two-dimensional electron gas concentrationcan be increased. In addition, it can be considered that the increase inthe channel layer thickness may lead to a decrease in energy at anexcited level of the two-dimensional electron gas, and thus may beeffective for improving the two-dimensional electron gas concentration.

However, it is difficult to increase the In composition of the InGaAsstrain layer and the thickness of the InGaAs layer while keeping afavorable crystal property of the InGaAs layer, because dislocationdefects may be developed due to the lattice misfit with the GaAs layer.Further, in any of the above described prior arts, values of thetwo-dimensional electron gas concentration and the electron mobility inthe PHEMT structure epitaxial substrate are not yet satisfactory in viewof possibility that the characteristics of electronic devices becomemore favorable with the increase in these values.

Therefore, in the PHEMT structure epitaxial substrate which usesn-AlGaAs as the electron supplying layer and an InGaAs layer as thechannel layer, it has strongly been desired to realize an epitaxialsubstrate having a higher two-dimensional electron gas concentration anda higher electron mobility compared with the currently reported values.

It is well known that the electron mobility is an important parameterfor improving various characteristics such as an on-resistance, amaximum current value, or a transconductance each of which is animportant performance indicator of a field effect transistor. Furtherimprovement of the electron mobility can achieve a reduction in thebuild-up resistance (on-resistance). This leads to a reduction in powerconsumption, so that an operating time of the battery can be prolonged.At the same time, since a calorific value can be decreased, it ispossible to realize higher integration of a device, and is also possibleto increase a degree of freedom of modular design by reducing a chipsize. From this viewpoint, it is desired to further improve the electronmobility in the case of a pHEMT which is used for various portableinstruments such as a cell phone.

DISCLOSURE OF THE INVENTION

The present inventors have devoted ourselves to solve the abovedescribed problems in the prior art and have consequently found that, ina pHEMT comprising an InGaAs strain channel layer and an AlGaAs electronsupplying layer containing n-type impurities, an emission peakwavelength in the InGaAs strain channel layer has a predeterminedcorrelation with an electron mobility thereat, and then the presentinventors have now achieved the present invention. That is, a highelectron mobility which has never been reported before has now realizedby laminating GaAs layers so as to respectively contact with a top and abottom surfaces of the InGaAs strain channel layer and then making thethickness thereof to a certain value or more for increasing an Incomposition of the channel layer, thereby obtaining an emissionwavelength from the channel layer of 1030 nm or more.

According to a first aspect of the present invention, there is provideda compound semiconductor epitaxial substrate used for a strain channelhigh electron mobility field effect transistor, comprising an InGaAslayer as a strain channel layer and an AlGaAs layer containing n-typeimpurities as an electron supplying layer, the above described InGaAshaving an emission peak wavelength layer at 77 K of 1030 nm or more.

According to a second aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovefirst aspect, wherein GaAs layers are provided as spacer layersrespectively in contact with a top surface and a bottom surface of theabove described InGaAs layer.

According to a third aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovesecond aspect, wherein each of the above described GaAs layers has athickness of 4 nm or more.

According to a fourth aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovefirst aspect, wherein the above described InGaAs layer has an electronmobility at 300 K of 8300 cm²/V·s or more.

According to a fifth aspect of the present invention, there is provideda method for manufacturing the compound semiconductor epitaxialsubstrate as described in the above first, second, third, or fourthaspect, comprising epitaxial growth of each compound semiconductor layerby employing a metalorganic chemical vapor deposition method (a MOCVDmethod).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of a layer structure showing one example of anembodiment of an epitaxial substrate according to the present invention;

FIG. 2 is a graph showing a relation between a mobility and an emissionpeak wavelength in the HEMT structure illustrated in FIG. 1;

FIG. 3 is a drawing which shows a layer structure of Example 1 of anepitaxial substrate according to the present invention;

FIG. 4 is a drawing which shows a layer structure of Example 2 of anepitaxial substrate according to the present invention;

FIG. 5 is a drawing of a layer structure of Comparative Example 1; and

FIG. 6 is a drawing of a layer structure of Comparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

One example of an embodiment of the present invention will now bedescribed in detail with reference to the drawings.

FIG. 1 is a drawing for describing one example of an embodiment of apHEMT structure epitaxial substrate according to the present invention.In FIG. 1, reference numeral 1 denotes a GaAs single crystal substrateand reference numeral 2 denotes a buffer layer formed on the GaAs singlecrystal substrate 1. Reference numeral 3 denotes a back side electronsupplying layer doped with n-type impurities formed as an n-AlGaAslayer, and a back side spacer layer (AlGaAs layer) 4 and a back sidespacer layer (GaAs layer) 5 are formed on the back side electronsupplying layer 3. Reference numeral 6 denotes a channel layer in whicha two-dimensional electron gas is formed for flowing two-dimensionalelectrons, and this layer is formed as an i-InGaAs layer having athickness from 4 nm to 13.5 nm depending on the In compositions thereof.

On the channel layer 6, a front side spacer layers 7 comprising a GaAslayer, a front side spacer layer 8 formed as an AlGaAs layer, a frontside electron supplying layer 9 formed as an n-AlGaAs layer, an undopedlayer (i-AlGaAs layer) 10, and another undoped layer (i-GaAs layer) 11are formed in this order.

Since the epitaxial substrate shown in FIG. 1 is formed as describedabove, electrons are supplied from the back side electron supplyinglayer 3 to the channel layer 6 through the back side spacer layers 4 and5, as well as supplying electrons from the front side electron supplyinglayer 9 to the channel layer 6 through the front side spacer layers 8and 7. Consequently, a high concentration of a two-dimensional electrongas is formed in the channel layer 6. The concentration of thetwo-dimensional electron gas within the channel layer 6 is significantlyinfluenced by the In composition of the channel layer 6 and by thethickness of the back side spacer layer 5 and the front side spacerlayer 7 being respectively contact with a top and a bottom surfaces ofthe channel layer 6. For the purpose of dramatically improving themobility of the two-dimensional electron gas compared with the priorart, various combinations of the In composition and the thickness of theupper and lower spacer layers were subjected to measurement forinvestigating a relation between an electron mobility in the channellayer 6 at room temperature and an emission peak wavelength at 77 K. Themeasurement results were as follows. In Composition Thickness ofMobility at Room Emission Peak of Channel Upper and Lower TemperatureWavelength Layer Space Layer (Å) (cm²/V · s) (nm) 0.20 20 7200 998 0.2040 7470 997 0.20 60 7790 996 0.30 40 8420 1061 0.30 60 8990 1068 0.35 608950 1075 0.40 60 8370 1064

The above described measurement result is plotted as shown in FIG. 2. Ascan be seen from FIG. 2, when an emission peak wavelength at 77 K is1030 nm or more, a value of an electron mobility is 8300 (cm²/V·s) whichis so extremely high that has never been reported before. Therefore, ifthe emission peak wavelength from the channel layer at 77 K is 1030 nmor more in the epitaxial substrate having a pHEMT structure which usesan i-InGaAs layer as the channel layer and an AlGaAs layer containingn-type impurities as the electron supplying layer as can be seen in FIG.1, the electron mobility can be dramatically increased.

Now there will be described a method for fabricating the epitaxialsubstrate which has the layer structure shown in FIG. 1. First, a GaAssingle crystal substrate 1 is prepared. The GaAs single crystalsubstrate 1 is a high-resistive semi-insulating GaAs single crystalsubstrate, and it is preferable to use a GaAs substrate manufactured bya LEC (Liquid Encapsulated Czochralski) method, a VB (VerticalBridgeman) method, a VGF (Vertical Gradient Freezing) method or thelike. In any of these manufacturing methods, it should be provided asubstrate having an angle of inclination from about 0.05° to 10° withrespect to one crystallographic plane direction.

A surface of the GaAs single crystal substrate 1 prepared as describedabove is degreased and washed, etched, rinsed, and dried, andsubsequently this single crystal substrate is placed on a heating tableof a crystal growth reactor. An inside of the reactor is substantiallysubstituted with high purity hydrogen before starting a heatingoperation. Once a moderate and stable temperature is reached within thereactor, arsenic source materials are introduced thereinto during thegrowth of a GaAs layer. Further, gallium source materials and aluminumsource materials are introduced thereto when an AlGaAs layer is grown,in addition to introducing the arsenic source materials thereto. When anInGaAs layer is grown, gallium source materials and indium sourcematerials are introduced thereto in addition to the introduction ofarsenic source materials. The desired laminated structure is being grownby controlling a supplying amount and a supplying time of each sourcematerial. Finally, the supply of respective source materials areterminated to stop the crystal growth, and after the cooling operation,the epitaxial substrate formed by the lamination as described in FIG. 1is removed from the reactor to complete the crystal growth. A substratetemperature at a time of growing crystals is usually from about 500° C.to 800° C.

The epitaxial substrate having the layer structure shown in FIG. 1 canbe fabricated by employing a MOCVD method. The advantage of employingthe MOCVD method is that the metalorganic compounds or hydrides ofatomic species can be used as the source materials.

Practically, arsenic trihydride (arsine) is generally used as an arsenicsource material during the epitaxial growth, however, it is alsopossible to use alkyl arsine which is obtained by substituting hydrogenof arsine with an alkyl group having 1 to 4 carbons. As gallium,aluminum, and indium source materials, it is generally used atrialkylate or a trihydride which is obtained by bonding an alkyl grouphaving 1 to 3 carbons or hydrogen to each metal atom.

As an n-type dopant, it is possible to use a hydride or an alkylatehaving an alkyl group whose carbon number is 1 to 3 of silicon,germanium, tin, sulfur, selenium or the like.

The present invention will now be described in detail below based on thecomparison of Examples with Comparative Examples, however the presentinvention is not limited to these examples.

EXAMPLE 1

A laminated structure shown in FIG. 3 was fabricated on asemi-insulating GaAs substrate by the epitaxial growth following the VGFmethod by the use of a low pressure barrel-typed MOCVD reactor.

In FIG. 3, reference numeral 11 denotes a GaAs substrate as a singlecrystal substrate, and reference numerals 12 to 15 respectively denotebuffer layers formed on the GaAs substrate 11. The buffer layers 12 to15 herein are respectively formed as an i-GaAs layer having a thicknessof 200 nm, an i-Al_(0.25)Ga_(0.75)As layer having a thickness of 250 nm,an i-GaAs layer having a thickness of 250 nm, and ani-Al_(0.25)Ga_(0.75)As layer having a thickness of 200 nm.

Reference numeral 16 denotes a back side electron supplying layer formedas an n-Al_(0.24)Ga_(0.76)As layer having a thickness of 4 nm and dopedwith n-type impurities at a concentration of 3×10¹⁸/cm³, and back sidespacer layers 17 and 18 are formed in this order on the back sideelectron supplying layer 16. In this case, the back side spacer layer 17is an i-Al_(0.24)Ga_(0.76)As layer having a thickness of 3 nm and theback side spacer layer 18 is an i-GaAs layer having a thickness of 6 nm.Reference numeral 19 denotes a channel layer in which a two-dimensionalelectron gas is formed, and is an i-In_(0.30)Ga_(0.70)As layer having athickness of 7.6 nm.

Each of reference numerals 20 and 21 is a front side spacer layer. Inthis case, the front side spacer layer 20 is an i-GaAs layer having athickness of 6 nm and the front side spacer layer 21 is ani-Al_(0.24)Ga_(0.76)As layer having a 3 nm.

Reference numeral 22 denotes a front side electron supplying layerformed as an n-Al_(0.24)Ga_(0.76)As layer having a thickness of 10 nmand doped with n-type impurities at a concentration of 3×10¹⁸/cm³. Bothof reference numerals 23 and 24 denote undoped layers and arerespectively an i-Al_(0.22)Ga_(0.78)As layer having a thickness of 3 nmand an i-GaAs layer having a thickness of 20 nm.

Trimethyl gallium (TMG) or trimethyl aluminum (TMI) was used as a sourcematerial of the third-group element, while arsine was used as a sourcematerial of the fifth-group element. Silicon was used as an n-typedopant, and the epitaxial growth was performed under the conditions thata pressure within the reactor was 0.1 atm, a growth temperature was 650°C., and a growth rate was 200 A/min to 300 A/min.

The channel layer 19 through which electrons run was epitaxially grownin order to obtain a strain InGaAs layer having an In composition of0.30 and a thickness of 7.6 nm. Further, undoped GaAs layers as spacerlayers were epitaxially grown to a thickness of 6.0 nm on a top surfaceand a bottom surface of the InGaAs layer used as the channel layer,respectively.

According to the laminated structure of Example 1 shown in FIG. 3 whichwas epitaxially grown as described above, the result of performing thehall measurement in accordance with a Van der Pauw method showed bettermeasurement values which had never been reported before, that is, thechannel layer 19 had a two-dimensional electron gas concentration of2.28×10¹²/cm² at room temperature (300 K), an electron mobility of 8990cm²/V·s at room temperature (300 K), a two-dimensional electron gasconcentration of 2.59×10¹²/cm² at 77 K, and an electron mobility of35600 cm²/V·s at 77 K. In addition, as a result of performing a CVmeasurement by using an Al schottky electrode with respect to thisstructure, a pinch-off voltage at a residual carrier concentration of1×10¹⁵/cm³ was −1.93 V.

In addition, a PL spectrum at 77 K was measured with respect to thelaminated structure of Example 1 as shown in FIG. 3. An emission peakwavelength from the channel layer 19 was 1068 nm.

EXAMPLE 2

A HEMT structure epitaxial substrate having a laminated structure shownin FIG. 4 was fabricated in a MOCVD reactor by using a GaAs substrate. Alayer structure shown in FIG. 4 is the same as a layer structure shownin FIG. 3 except only that the channel layer 31 has an In composition of0.35 and a Ga composition of 0.65 as well as having a thickness of 5.5nm. Therefore, layers in FIG. 4 corresponding to layers in FIG. 3 arerespectively marked with the like reference numerals, and overlappeddescriptions of these layers are omitted.

As for the laminated structure shown in FIG. 4 which was epitaxiallygrown under the same conditions as in the case of Example 1, the resultof performing the hall measurement in accordance with a Van der Pauwmethod showed better measurement values, that is, the channel layer 31had a two-dimensional electron gas concentration of 2.22×10¹²/cm² atroom temperature (300 K), an electron mobility of 8950 cm²/V·s at roomtemperature (300 K), a two-dimensional electron gas concentration of2.50×10¹²/cm² at 77 K, and an electron mobility of 33000 cm²/V·s at 77K. In addition, as a result of performing a CV measurement by using anAl schottky electrode with respect to this structure, a pinch-offvoltage at a residual carrier concentration of 1×10¹⁵/cm³ was −1.75 V.

In addition, a PL spectrum at 77 K was measured with respect to thelaminated structure of Example 2 as shown in FIG. 4. An emission peakwavelength from the channel layer was 1075 nm.

COMPARATIVE EXAMPLE 1

A HEMT structure epitaxial substrate having a laminated structure shownin FIG. 5 was fabricated as Comparative Example 1 in a MOCVD reactor byusing a GaAs substrate. A layer structure shown in FIG. 5 is the same asa layer structure of Example 1 shown in FIG. 3 except only that thechannel layer 19A has an In composition of 0.20 and a Ga composition of0.80 as well as having a thickness of 13.5 nm, and that each of i-GaAslayers 18A and 20A as the back side and front side spacer layers has athickness of 2 nm and the undoped layer 23A has a thickness of 7 nm.Therefore, layers in FIG. 5 corresponding to layers in FIG. 3 arerespectively marked with the like reference numerals, and overlappeddescriptions of these layers are omitted.

As for the laminated structure of Comparative Example 1 shown in FIG. 5which was epitaxially grown under the same conditions as in the case ofExample 1, the result of performing the hall measurement in accordancewith a Van der Pauw method was as follows, that is, the channel layer19A had a two-dimensional electron gas concentration of 2.55×10¹²/cm² atroom temperature (300 K), an electron mobility of 7200 cm²/V·s at roomtemperature (300 K), a two-dimensional electron gas concentration of2.78×10¹²/cm² at 77 K, and an electron mobility of 21900 cm²/V·s at 77K. These values were similar to the conventionally reported values. Inaddition, as a result of performing a CV measurement by using an Alschottky electrode with respect to this structure, a pinch-off voltageat a residual carrier concentration of 1×10¹⁵/cm³ was −2.12 V.

In addition, a PL spectrum at 77 K was measured with respect to thelaminated structure of Comparative Example 1 shown in FIG. 5. Anemission peak wavelength from the channel layer was 998 nm.

COMPARATIVE EXAMPLE 2

A HEMT structure epitaxial substrate having a laminated structure shownin FIG. 6 was fabricated as Comparative Example 2 in a MOCVD reactor byusing a GaAs substrate. A layer structure shown in FIG. 6 is the same asa layer structure of Example 2 shown in FIG. 4 except only that thechannel layer 31A has an In composition of 0.20 and a Ga composition of0.80 as well as having a thickness of 13.5 nm, and that the none-dopedlayer 23A has a thickness of 7 nm. Therefore, layers in FIG. 6corresponding to layers in FIG. 4 are respectively marked with the likereference numerals, and overlapped descriptions of these layers areomitted.

As for the laminated structure of Comparative Example 2 shown in FIG. 6which was epitaxially grown under the same conditions as in the case ofExample 2, the result of performing the hall measurement in accordancewith a Van der Pauw method was as follows, that is, the channel layer31A had a two-dimensional electron gas concentration of 2.19×10¹²/cm² atroom temperature (300 K), an electron mobility of 7790 cm²/V·s at roomtemperature (300 K), a two-dimensional electron gas concentration of2.44×10¹²/cm² at 77 K, and an electron mobility of 30800 cm²/V·s at 77K. These values were similar to the conventionally reported values. Inaddition, as a result of performing a CV measurement by using an Alschottky electrode with respect to this structure, a pinch-off voltageat a residual carrier concentration of 1×10¹⁵/cm³ was −1.90 V.

In addition, a PL spectrum at 77 K was measured with respect to thelaminated structure of Comparative Example 2 shown in FIG. 6. Anemission peak wavelength from the channel layer was 996 nm.

It has been confirmed by the results from Examples 1, 2 and ComparativeExamples 1, 2 that, in the PHEMT structure epitaxial substrate, anelectron mobility at room temperature (300 K) can be increased to 8300cm²/V·s or more by forming a GaAs layer on each of a top surface and abottom surface of an InGaAs channel layer to a thickness of 4.0 nm ormore, increasing an In composition of the channel layer, and adjusting aPL emission wavelength from the InGaAs channel layer to 1030 mm or more.

It is considered that an increase in thickness of the GaAs layerlaminated on each of a top surface and a bottom surface of the InGaAschannel layer reduces irregularities of an interface between the InGaAslayer and the AlGaAs layer, which leads to prevention of the reductionin electron mobility due to the scattering caused by the irregularitiesof the interface. At the same time, it is also considered thatthree-dimensional growth at a surface of the InGaAs layer due tosegregation of In is prevented, which has an effect of preventing thescattering at the interface as well. In addition, when a GaAs singlecrystal substrate which is less-dislocated with respect to a VGFsubstrate or a VB substrate is used, increase in a critical thickness ofthe InGaAs layer prevents the development of dislocation defects causedby a lattice misfit with the GaAs layer, therefore, this has an effectof increasing a layer thickness while keeping a favorable crystalcharacteristic of the InGaAs layer.

A peak wavelength of an emission spectrum from the channel layer (InGaAslayer) depends on an In composition of the channel layer and a thicknessof the channel layer. A band gap becomes narrower with increasing the Incomposition, and the peak wavelength shifts toward a long wavelengthside. In addition, an excitation level becomes lower with increasing thechannel layer thickness, and also the peak wavelength shifts toward thelong wavelength side. Therefore, the peak wavelength of the emissionspectrum can be used as evaluation means for optimizing the Incomposition and thickness of the channel layer simultaneously.

Each of the layer structures of the epitaxial substrates from Examples1, 2 and Comparative Examples 1, 2 is used as a test structure forevaluating the mobility following the Hall measurement and forevaluating the two-dimensional electron gas characteristics such as thethreshold voltage measurement following the CV measurement.

In a layer structure of an actual epitaxial substrate used forfabricating a FET device, a thickness of a layer corresponding to thenone-doped GaAs layer 14 of the epitaxial substrate from Examples 1, 2and Comparative Example 1, 2 is increased, and in addition, contactlayers are laminated for achieving ohmic contacts with a sourceelectrode and a drain electrode. As the contact layer, it is usuallyused an n-GaAs layer doped with silicon at a concentration about 3×10¹⁸to 5×10¹⁸/cm³ and laminated to a thickness of about 100 nm.

However, the effect of improving the mobility according to the presentinvention is never compromised by a process for growing the contactlayers and fabricating the FET device. The effect of improving themobility according to the present invention is significant not only inthe test structures for evaluating the characteristics of epitaxialsubstrates from Examples 1, 2 and Comparative Examples 1, 2, but also ina structure of an epitaxial substrate used for the FET device.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided a pHEMT structureepitaxial substrate having an electron mobility and a two-dimensionalelectron gas concentration which have never been reported before andbeing advantageous to the fabrication of electronic devices.

1. A compound semiconductor epitaxial substrate for use in a strainchannel high electron mobility field effect transistor, comprising anInGaAs layer as a strain channel layer and an AlGaAs layer containingn-type impurities as an electron supplying layer, wherein said InGaAslayer has an emission peak wavelength at 77 K of 1030 nm or more.
 2. Thecompound semiconductor epitaxial substrate according to claim 1, whereinGaAs layers are provided as spacer layers in contact with a top surfaceand a bottom surface of said InGaAs layer, respectively.
 3. The compoundsemiconductor epitaxial substrate according to claim 2, wherein each ofsaid GaAs layers has a thickness of 4 nm or more.
 4. The compoundsemiconductor epitaxial substrate according to claim 1, wherein saidInGaAs layer has an electron mobility at 300 K of 8300 cm²/V·s or more.5. A method for manufacturing the compound semiconductor epitaxialsubstrate according to claim 1, 2, 3, or 4, comprising epitaxiallygrowing each compound semiconductor layer by employing a metalorganicchemical vapor deposition (MOCVD) method.